Using volume buttons to control speed?

I am making a VR app where the user can look around and control movement speed up and down.

As it is hard to control the speed using the touchscreen while the phone is headmounted, we want to use the volume keys on the headset to do that.

Apparently it is possible to register the button presses using some private APIs in iOS, but just reading the current volume would also be OK and set the speed to that.

Is there any easy way to do this in Unity? Any existing plugins or built in functionality that would provide the keypresses or current sound volume?

Unfortunately I am not into iOS plugin development, so it would be great if there is some way to acheive this already...

 

This is not implemented in Unity because of the fact what Apple refuses apps with such behaviour.